Plant nutrients have varying amounts of mobility. Some nutrients, once absorbed by the roots through the soil or through the leaves via fertilizing foliar sprays, are transported to a fixed position in the plant. Others move around the plant where they are needed. The most visible example of this is when the plant shows deficiency in mobile nutrients. The deficiency is evidenced in the older leaves first because the plant moves mobile nutrients from older leaves to younger ones.
Nitrogen is a primary plant nutrient that helps a plant synthesize proteins and amino acids. It is one of the most mobile plant nutrients. It is so mobile, in fact, that unabsorbed nitrogen from over-fertilization often washes away and contaminates surface and groundwater supplies.
When a plant is nitrogen-deficient, the leaves lower down on the plant will begin to yellow as the plant steals nitrogen from them to provide the actively growing leaves with the mobile nutrients they need.
Magnesium is a secondary plant nutrient that aids in the plant's manufacture of chlorophyll. Magnesium deficiency is common in coarse soils, acidic soils, and soils with too much potassium. The older, lower leaves of plants that are deficient in magnesium will develop yellow streaks between their veins as the plant moves the magnesium to younger leaves.
Phosphorus is a primary macronutrient that the plant needs to develop nucleic acids and adenosine triphosphate (ATP) essential to the plant's metabolic functions.
Older leaves in phosphorus-deficient plants may develop a purple, yellow or dull-green color, depending on the species. Young plants may become stunted. Older plants with severe deficiencies may have delayed development.
Potassium is a primary plant macronutrient. This mobile nutrient aids in intercellular chemical reactions and ion transport. The older leaves in plants with potassium deficiency turn yellow around the margins, eventually develop brown spots and die. The plant may also have difficulty producing fruit, or wilt even though it is given sufficient water.
Chlorine is a mobile micronutrient that aids the plant in photosynthesis. The physical symptoms of chlorine deficiencies are difficult to isolate because a plant that is calcium-deficient is likely also deficient in other micronutrients.
Zinc is a mobile micronutrient that activates enzymes in the plant. Deficiency in this micronutrient affects the young leaves. Without sufficient amounts of this micronutrient, leaves may develop yellowing between their veins or their growth may be stunted.